Ionophores, or ion-bearers, as reported in Ann. N.Y. Acad. Scil, Vol. 264, pp. 373-86 (1985), are polyether antibiotics that modulate the physiological transport of ions across biological membranes and alter the characteristics of fermentation in the animal, resulting in favorable metabolic changes. These valuable properties of ionophores led to their use as feed additives by the livestock industry. For example, the ionophores, when fed to ruminants, resulted in an improved feed-gain ratio, as reported in Feedstuffs, pp. 14, 15 and 22 (1989). In one accepted use, ionophores are fed to feedlot cattle in confinement for improved feed efficiency. In this use, the ionophore first is mixed with a finely ground nonmedicated feedstuff to produce a premix, which premix is added to an air-dry feed for feeding to cattle, including steers and heifers.
While the above described prior art use of ionophore results in improved feed efficiency, usually of from 5 to 8 percent or higher, for steers and heifers, as reported in Feedstuffs (supra), serious shortcomings accompany this use. For example, since the ionophore is mixed with feed, one shortcoming is the difficulty to ascertain the amount of ionophore ingested by the animal because of feedlot losses such as spillage and scatter. Another shortcoming resides in the absence of controlled administration of known amounts of the ionophore over time, as the composition of the feed charged with the ionophore can vary with feed millers. Also, ionophores are sensitive to moisture in the environment, which moisture can adversely affect their usefulness, and the handling and transport of feeds containing ionophores can result in the segregation of particles carrying ionophores and change the concentration level to which cattle are exposed when fed over time. Then, since ionophores usually are mixed with feeds daily, this requires extra labor that adds to the cost of the ionophore-feedstuff.
In the light of the above presentation, it will be appreciated by those versed in the dispensing art to which this invention pertains that a pressing need exists for a dosage form that can deliver a valuable ionophore to a biological environment of use comprising livestock for both improved feed efficiency and the enhancement of growth of the livestock. The pressing need exists also for a dosage form that can store and deliver an ionophore at a controlled rate in a substantially constant dose per unit time over a prolonged period of time essentially independent of the environment of use, which environment of use pertains to livestock that are confined and to livestock in the pasture. It will be appreciated further by those versed in the dispensing art that if such a novel and unique dosage form is provided that can administer an ionophore in a rate-controlled dose over time and, simultaneously, provide the beneficial effects, the dosage form would represent an advancement and valuable contribution in the ionophore dosage form art.